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List:       sip-implementors
Subject:    Re: [Sip-implementors] Send media in video call Vs audio call in SDP Answer/Offer model
From:       Amith Nambiar <amith () ctg ! ae>
Date:       2008-12-16 6:48:22
Message-ID: 0KBY00HOKIREWNK0 () apmail2 ! emirates ! net ! ae
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----- Original Message  -----
From: Richard 

Hi all,

    Suppose caller A wants to initiate a video call with B. He sends an 
INVITE to B and B accepts the call and then sends back 200 OK with SDP. 
According to RFC 3264, practically caller B will send the audio and 
video RTP packet to caller A immediately. Since SIP messages and RTP 
packets take different paths to the destination, the media data usually 
arrive to caller A first before the 200OK. It's okay for audio data 
because voice decoder can decode individual RTP payloads even the first 
few packets are skipped (e.g. G.711, G.729). However, it's not case in 
video. Take H.263 as an example. Usually the first encoded video frame 
is I-frame, and the next few frames are P-frames. If the first I-frame 
are skipped, caller A cannot decode the video packet properly until he 
gets next I-frames. There would cause significant delays for caller A to 
see the caller B. How to solve it? 

Hi Richard,
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&n \
bsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; \
For requesting an I-frame there is a mechanism mentioned in the draft below. But the \
problem is whether the UA's involved have implemented them or not.  I have seen a \
Media Gateway from a well known company&nbsp; using the mechanism mentioned in  \
http://tools.ietf.org/html/draft-levin-mmusic-xml-media-control-13  for getting the \
Intra frame from the Media Server. I have no idea how many UA's use this mechanism. \
But looks like this draft addresses your problem. good luck.


Best,
Amith 


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